Triad Satellite Research Articles

Determination of the Volland convection electric field parameters and computation of the associated field-aligned current distribution

The data obtained by some ground-based and spacecraft observations, especially by GEOS-2, are gathered to give experimental parameters which can be introduced in the Volland model of the magnetospheric convection electric field. Two models are used: the single boundary layer model and the double boundary model. With these models, the field-aligned current distributions over the auroral zone are calculated for two different levels of geomagnetic activity. The results of the calculations are roughly consistent both for the amplitude and the local time variation with the observations made with the TRIAD satellite (Iijima and Potemra, 1978, J. geophys. Res. 83, 599), especially for region I currents. This demonstrates the validity of the revised Volland model and of the dynamical plasma density model at the geostationary orbit which has been deduced from the GEOS-2 relaxation sounder.

Polar convection and Birkeland Currents during strongly positive IMF By

The TRIAD satellite passed directly over the field of view of the Scandinavian Twin Auroral Radar Experiment (STARE) radar on May 1, 1978, at approximately 0618 UT when the By component of the interplanetary magnetic field (IMF) was unusually large in comparison to the other two components. The average IMF values in the interval from 0600 to 0700 UT on this day were Bx = −1.8 nT, By = 8.7 nT, and Bz = −0.2 nT (King, 1979). The TRIAD magnetic field perturbations revealed the presence of a very intense (ΔB ∼ 500 nT) region 1 downward‐flowing Birkeland current near 0930 MLT and poleward of a much weaker upward‐flowing region 2 current. This “net” (unbalanced) Birkeland current was observed at a 67° magnetic latitude in this morning sector (near 0930 MLT). The net Birkeland current was located equatorward of a region of intense westward (antisunward) ionospheric convection flow detected by the STARE radar. We conclude that the polar cap convection flow is distorted and displaced to low latitudes (< 67° magnetic latitude) in the morning sector during this period of strongly positive By and almost negligible Bx and Bz. The large net region 1 Birkeland current is associated with the convection flow reversal. Convection velocity data acquired by the AE‐C satellite on October 29, 1978, near 1723 UT when Bx = −0.5 nT, By = 8.3 nT, and Bz = −2.7 nT (average values during the 1700 to 1800 UT interval) show the convection reversal near 67° invariant latitude and 0800 MLT (King, 1979). These two examples support previous suggestions for the important influence that the By component of IMF has on the intensity and location of high‐latitude convection and Birkeland currents.

Auroral zone conductivities within the field‐aligned current sheets

We have used field‐aligned current data obtained by the Triad satellite during 21 passes over Alaska to order conductivity measurements made simultaneously by the Chatanika radar. Three distinct field‐aligned current regions were sampled during these experiments: the region 2 current sheet in the morning and evening sectors and the region 1 current sheet in the evening sector. The conductances in the region 2 evening sector current sheet increase uniformly with latitude, and the total increase in conductance across the sheet is fairly constant. The latitudinal profiles of conductance across the region 1 evening sector current sheet show that auroral arcs tend to occur in the poleward portion of the sheet. In the morning sector region 2 current sheet the Hall conductance is larger and changes much more rapidly with latitude than the Pedersen conductance. The Hall and Pedersen conductance can be interpreted in terms of the number density and temperature of a Maxwellian distribution of electrons that would produce the same conductance values. We call this electron population the equivalent Maxwellian source. The temperatures and densities thus deduced are similar to those of the electrons in the plasma sheet. Conductances in the region 2 evening sector current sheet indicate a source with a temperature between 0.5 and 2 keV and density between 0.1 and 1 cm−3. The morning sector diffuse aurora within the region 2 current sheet originates from a population with temperatures between 1 and 4 keV and density between 0.6 and 2 cm−3. In auroral arcs the conductances are consistent with those expected from plasma sheet electrons that have been accelerated along magnetic field lines. The concept of equivalent Maxwellian source is also used to study the extent to which the precipitating electrons can carry the field‐aligned current in the various regions.

Aurora and electrojet configuration in the early morning sector

Nearly simultaneous data from the Chatanika radar, the TRIAD satellite, and the IMS Alaska meridian chain of magnetic and auroral observatories are utilized to examine the spatial relationship among auroras, field‐aligned currents, ionospheric electric fields, and conductivities in the westward electrojet in early morning hours. The westward electrojet is found to be dominated by one of two different quantities, depending on latitudinal locations in this local time sector. The poleward half of the electrojet is governed by a relatively intense southward electric field. On the other hand the Hall conductivity is the dominant factor in controlling the electrojet current in the equatorward half. The electrojet center appears to be located within the region of the upward field‐aligned current. Major auroral activity is present only in the equatorward half of the electrojet in which the upward field‐aligned current prevails, although it appears that the latitudinal variations of the conductivity and current intensity are not well related within that region.

Relationship between field‐aligned currents, diffuse auroral precipitation and the westward electrojet in the early morning sector

On the basis of Chatanika radar data and simultaneous Triad satellite data, the relationships between field‐aligned currents, diffuse aurora, electric fields, and currents in the early morning sector are examined. The diffuse aurora is coincident with the upward region 2 field‐aligned current. Within the upward region 2 field‐aligned current, small‐scale current structures are often present. These structures seem to coincide with small‐scale electron density enhancements in the E region, which are probably produced by energetic electron precipitation. The westward electrojet extends across the region 2/region 1 boundary in the morning sector. The southward electric field is large at and poleward of this boundary. It is most probable that the region 2 field‐aligned currents are closed by southward Pedersen currents within the ionosphere.

Recent results in auroral-zone scintillation studies

Nighttime data from the Defense Nuclear Agency's Wideband satellite have consistently shown a pronounced scintillation enhancement at the point where the propagation vector lies within the local L-shell. Simultaneous observations at two well-separated stations and spaced-receiver measurements have shown that this feature is caused by a latitudinally narrow, unstable F-region ionization enhancement that produces sheet-like intermediate-scale structures. A more detailed characterization of the source region has been derived from simultaneous measurements from the TRIAD satellite and the Chatanika radar. The source region is typically convecting southward, whereby the poleward gradient at its southern boundary is stable to the gradient-drift instability. Thus, Ossakow and Chaturvedi (1979) postulated the current-convective instability as a source mechanism. Birkeland currents, when sufficiently intense, can destabilize a region that is otherwise stable to the gradient-drift instability. The sheet-like anisotropy of the irregularities is perhaps the most intriguing feature of the instability. Two mechanisms have been postulated to explain it. This paper reviews the development and current status of our understanding of this recently discovered high-latitude instability.

Field-aligned currents and magnetic disturbances in the dayside polar region

Field-aligned currents in the day-time cusp-region are regarded as the superposition of various current sheets under the influence of different solar wind parameters. The principal feature of this pattern is a specific region 3 of field-aligned currents located poleward of region 1 and affected by both the azimuthal and northward components of the interplanetary magnetic field. It is shown that recent measurements carried out on the TRIAD satellite (Saflekos and Potemra, 1979) unambiguously point to the existence of region 3. The data on the transverse magnetic disturbances supplied by S3-2 satellite (Doyle et al., 1981) accord with our model on taking into account the relations between the IMF parameters and the field-aligned current intensity.

The relationship between interplanetary quantities and Birkeland current densities

We have examined the relationship between hourly values of solar wind density, speed and the interplanetary magnetic field and the densities of Region 1 Birkeland currents. Magnetic field observations acquired by the TRIAD satellite were used to determine the Birkeland current densities. Data acquired from 51 passes in the morning sector (0800 to 1300 MLT) during IMF By > 0 and 74 passes in the afternoon sector (1100 to 1600 MLT) during IMF By < 0 were used in this study. This separation of data by polarity of IMF By was done to avoid the complicated Birkeland current patterns related to the cusp region. To avoid further complications involving the seasonal dependence of Birkeland currents (c.f. Fujii et al., 1981), north polar data were selected during the period from May to August in 1973, 1974, 1976, and 1977. Correlation coefficients were computed for the morning and afternoon Region 1 densities (Jm and Ja, respectively) related to a variety of combinations of solar wind density (N), speed (V), and IMF By and Bz. The current densities show good correlation with negative values of Bz (R = 0.39 for Jm and R = 0.56 for Ja), but relatively poor correlation with positive values of Bz (R = 0.07 for Jm and R = 0.1 for Ja). Relating Jm and Ja with BT = (By² + Bz²)½ provides correlation coefficients of 0.50 and 0.48; with BT sin (θ/2), where θ is measured from the positive z axis, provides coefficients of 0.56 and 0.68; with the Akasofu “ϵ” parameter provides coefficients of 0.39 and 0.67; with VBT sin (θ/2) gives 0.52 and 0.74; with [N½ VBT sin (θ/2)]½ = (NVVA)½ gives 0.66 and 0.75. The dominant factor in these correlations appears to involve the magnitude of the IMF in the y‐z plane (BT) and its orientation with respect to the geomagnetic field, sin (θ/2). This suggests that magnetic reconnection may play an important role in determining the strength of Region 1 Birkeland currents. Substantial current densities (1 to 2 µA/m²) exist at the origin of these plots (i.e. for θ = 0). These persistent Birkeland currents may be related to the convective flow (and associated electric fields) which prevail during periods of northward IMF.

Correlated Birkeland current signatures from the Triad and MAGSAT magnetic field data

On November 1, 1979, the MAGSAT satellite was successfully launched into a 400 km polar orbit from which it could measure the earth’s magnetic field. During the first part of MAGSAT’s eight‐month lifetime, the orbit was nearly coplanar with that of the TRIAD satellite at 800 km altitude. This study has concentrated on transverse magnetic disturbances, associated with Birkeland currents, measured by the two spacecraft. The satellites were nearly coplanar from November through the middle of December 1979. Data were compared when the satellites were over the TRIAD/Chatanika receiving station. Of the approximately 150 comparisons of magnetic disturbances, 75% are very similar in shape and magnitude. For these cases, the TRIAD peak disturbances were, on the average, larger than the peak MAGSAT disturbances by 7%, which is less than many uncertainties in these measurements (due to baselines, calibration). Better agreement in the large‐scale well‐defined field‐aligned currrent signatures occurs during periods of higher Kp even with time separations as large as 45 minutes. The discrepancies in the remaining cases may be due to temporal, longitudinal and height effects of the Birkeland current system.

Field‐aligned currents in high latitudes estimated from Millstone Hill Radar observations of ion drifts

A simple scheme has been developed to estimate the global distribution of field‐aligned currents by using electric field measurements by an incoherent scatter radar at Millstone Hill. The combination of the electric field data and a realistic, quiet‐time model of the ionospheric conductivity gives horizontal currents in the ionosphere. The divergence of the ionospheric currents then yields the distribution of the field‐aligned currents. This method is applied to two quiet days in May and July 1978. The field‐aligned currents obtained are consistent with TRIAD satellite observations. The peak location of the global field‐aligned current intensity and the spatial distribution of field‐aligned currents near midnight appear to vary between these two magnetically quiet days.

Field‐aligned currents associated with Pc 5 pulsations: STARE and TRIAD observations

The resonance theory for monochromatic Pc 5 micropulsations predicts that the toroidal mode resonance should be accompanied by field‐aligned current systems. These currents close via Pedersen current in the ionosphere, and the drifting irregularities observed by the STARE radars are thought to be generated by the associated electric fields. Recent theoretical calculations by Walker (1980) are used to estimate the micropulsation current systems and accompanying magnetic perturbations. STARE radar observations of the pulsation electric field in the ionosphere are then compared with the corresponding magnetic perturbation observed with the TRIAD satellite magnetometer.

Electrodynamic properties of the evening sector ionosphere within the region 2 field‐aligned current sheet

Chatanika radar measurements were combined with magnetometer data from the Triad satellite to relate the electrodynamic properties of the ionosphere to field‐aligned currents. Radar electric field and electron density data were studied for eight evenings during which the region 2 (downward) field‐aligned current was entirely within the radar field of view. In all cases the downward current region coincided with an E region electron density that increased to the north. The equatorward boundary of the current sheet was associated with maximum E region densities of about 1 × 105 el/cm³. The poleward boundary of the current sheet was always characterized by a local maximum in E region ionization referred to as the interface arc. The ionospheric perpendicular current was generally northeastward within the downward current sheet. The gradient in the northward ionospheric current was used to compute field‐aligned currents, which were in good agreement with those measured by Triad.

Seasonal dependence of large‐scale Birkeland currents

The seasonal dependence of large‐scale Birkeland currents has been determined from the analysis of vector magnetic field data acquired by the TRIAD satellite in the northern hemisphere. Statistical characteristics of single sheet (i.e., net currents) and double sheet Birkeland currents were determined from 555 TRIAD passes during the summer, and 408 passes during the winter (more complicated multiple‐sheet current systems were not included in this study). The average Kp value for the summer events is 1.9 and for the winter events is 2.0. The principal results include the following: (1) The single sheet Birkeland currents are statistically observed more often than the double sheet currents in the dayside of the auroral zone during any season. The single sheet currents are also observed more often in the summer than in the winter (as much as 2 to 3 times as often depending upon the MLT sector). (2) The intensities of the single and double sheet Birkeland currents on the dayside, from approximately 1000 MLT to 1800 MLT, are larger during the summer (in comparison to winter) by a factor of about 2. (3) The intensities of the double sheet Birkeland currents in the nightside (the dominant system in this local time) do not show a significant difference from summer to winter. (4) The single and double sheet currents in the dayside (between 0600 and 1800 MLT) appear at higher latitudes (by about 1° to 3°) during the summer in comparison to the winter. These characteristics suggest that the Birkeland current intensities are controlled by the ionospheric conductivity in the polar region. The greater occurrence of single sheet Birkeland currents during the summertime supports the suggestion that these currents close via the polar cap when the conductivity there is sufficiently high to permit it. Since the intensities of Birkeland currents are larger during periods of greater ionospheric conductivity, an important source (but perhaps not the only source) of these currents must be a voltage generator in the magnetosphere, possibly related to the convective electric field.

Distant magnetic field effects associated with Birkeland currents (made possible by the evaluation of TRIAD's attitude oscillations)

The magnetic field data acquired by the TRIAD satellite sometimes show variations with periods longer than 10 min and with amplitudes of a few thousand nanoTeslas due to attitude oscillations. From a study of 150 passes, the principal oscillations of TRIAD have been identified as the well‐known librations of a gravity‐stabilized satellite. The libration periods are approximately T0/2 and T0/(3)1/2, where T0 is the orbit period of about 100 min. The amplitude and phase of these librations appear to change over periods of a few days, and sometimes they vanish altogether. By use of data acquired over several consecutive TRIAD orbits, these attitude variations can be numerically evaluated and removed, leaving the disturbances associated with Birkeland currents. The establishment of a ‘base line’ for the TRIAD observations by using this technique has permitted, for the first time, a preliminary evaluation of the ‘current‐free’ magnetic field disturbance due to large‐scale Birkeland currents. Data acquired from three consecutive TRIAD passes (spanning more than 3 hours) show nearly the same magnetic disturbance profile, which extends as far as 10 degrees in latitude from a single (net) region 1 Birkeland current sheet. These observations confirm the permanent and global nature of large‐scale Birkeland currents.

Chatanika/Triad observations of unstable ionization enhancements in the auroral F‐region

Preliminary results are presented from a campaign of coordinated measurements between the Chatanika radar and the TRIAD satellite to investigate production mechanisms responsible for localized high‐latitude scintillations. The radar measured the latitudinal variations of plasma density and electric field while the satellite measured the field‐aligned current distribution with latitude. This information was used to calculate the linear growth rate of the current convective instability. We find that field‐aligned ionization enhancements, which are a common feature of the auroral F‐region, are unstable, with instability growth times of ≤ 6 minutes. The observed plasma configuration, which was already gradient drift unstable on its poleward edge, was further destabilized by a field‐aligned current of ∼ 0.8 µA/m². The presence of electron density irregularities produced in the unstable region was verified by observing VHF scintillations on the TRIAD telemetry signal.

The orientation of birkeland current sheets in the dayside polar region and its relationship to the IMF

Vector magnetic field observations made with the three‐axes magnetometer on the Triad satellite have been used to study the orientation of magnetic disturbances in the dayside polar region. These measurements were all made over the southern polar region and recorded at McMurdo, Antarctica. These disturbances are transverse to the main geomagnetic field and may be interpreted as being caused by field‐aligned Birkeland current sheets consistent with Maxwell's equations. The current sheets in the regions usually associated with the morning and afternoon auroral regions are most often aligned in the geomagnetic east‐west direction. The amplitudes of these ‘south auroral’ currents are larger in the morning than in the afternoon when the interplanetary magnetic field (IMF) is directed toward the sun (By < 0, Bx >0) and larger in the afternoon when the IMF is directed away (By > 0, Bx <0). This is the systematically reversed relationship between Birkeland current intensities and IMF found in the northern auroral zone by McDiarmid et al. (1977). The Birkeland current sheets in the region associated with the southern cusp are also aligned in the geomagnetic east‐west direction during periods of negative Bz. During periods of By <0 and Bx > 0 the Birkeland current flow in the region of the southern cusp is predominantly away from the ionosphere in contrast to the downward flow into the northern cusp as determined earlier (e.g., McDiarmid et al., 1978b; Iijima et al., 1978). The cusp Birkeland current flow directions appear to reverse for By > 0 and Bx < 0. From a search of the Triad data set, some rare examples of magnetic disturbances with a large north‐south (noon‐midnight) component have been discovered in the polar cap near noon. These always occurred during periods of northward IMF. These disturbances have been interpreted as being caused by sun‐aligned Birkeland current sheets. The intensities of these currents in a few cases studied here are correlated with the positive amplitude of Bz and are comparable to the cusp and auroral currents studied earlier (0.1–0.8 A/m). The fluxes of electrons associated with sun‐aligned polar arcs measured with the Isis 2 satellite (Ismail et al., 1977) are sufficiently intense to produce upward flowing sun‐aligned Birkeland current sheets, but the origin of the downward flowing currents is not clear. These sun‐aligned current sheets may be associated with the rare occurrence of multiple‐cell convective flow patterns in the polar cap (Burke et al., 1979; Spiro et al., 1979).

Day‐to‐day and average magnetic variations along the IMS Alaska Meridian Chain of observatories and modeling of a three‐dimensional current system

The daily magnetic variations above 60° in magnetic latitude are studied by using records from the International Magnetospheric Study Alaska meridian chain of observatories. Several days with activity varying from moderate to disturbed are chosen to study day‐to‐day variations, namely, the daily variability of polar magnetic variations. The average daily magnetic field variations and the equivalent current pattern are also obtained by using the successive records between March 9 and April 27, 1978. They represent the disturbance condition on a moderately disturbed day (the average ΣKp value during this period was 23o). The equivalent current pattern reveals clearly the large‐scale features, in particular the so‐called ‘two‐cell’ pattern. Then, the distribution of the ionospheric currents and the field‐aligned current are computed on the basis of the average daily magnetic field variations. It is shown that the computed ionospheric current distribution has little resemblance to the two‐cell pattern. It is concluded that the main part of the ionospheric currents must be fed by field‐aligned currents with a distribution similar to that inferred from Triad satellite data.

Stare and Triad observations of field‐aligned current closure and Joule heating in the vicinity of the Harang discontinuity

By combining data from the Stare auroral radar system and the Triad satellite, it has been possible to study the nature of field‐aligned current closure in the vicinity of the Harang discontinuity. For the 2 days studied the largest electric fields were directed predominantly in the north/south directions, and field‐aligned current closure was via north/south directed Pedersen currents. The concurrent electrojets were overlapping east/west directed Hall currents with the westward electrojet to the north. From the Stare‐Triad data, latitudinal profiles of the ionospheric Joule heating rate and the height‐integrated Pedersen conductivity have been calculated. The Joule heating rate maximized to the north and south of an auroral arc and had peak values of 2–4 mW/m². The height‐integrated Pedersen conductivity outside of the auroral arc was approximately 4Ω−1, whereas in the vicinity of the arc, it was in excess of 12Ω−1. These values are in good agreement with incoherent scatter and rocket measurements. The agreement is gratifying, since our technique for the calculation of the Pedersen conductivity does not require a model of the altitude‐dependent collision frequency.

Nearly simultaneous observations of field‐aligned currents and visible auroras by the Triad and Isis 2 satellites

Magnetic field data from the Triad satellite at 800‐km altitude are utilized to define the poleward and equatorward boundaries of field‐aligned currents in the premidnight sector when auroral luminosity along the same meridian was scanned by another polar‐orbiting satellite, Isis 2. It is found that the latitudinal boundaries of the major portion of the field‐aligned currents line up very well with the auroral luminosity boundaries at both the poleward and equatorward sides of the auroral distribution. The boundary between the upward and downward field‐aligned currents generally occurs at a minimum in the auroral luminosity profiles. Poleward of this minimum, the current is upward, and the aurora is characterized by the discrete auroral component with intensities at the boundary of 1–2 kR. Equatorward, in the region of the downward current, the aurora is essentially diffuse, and the auroral intensity at the boundary is 0.5–1 kR.

The relationship between ionospheric and field‐aligned currents in the dayside cusp

Observations of field‐aligned currents determined from magnetic disturbances measured with the Triad satellite near local magnetic noon have been compared to simultaneous magnetic perturbations observed at the ground with a dense chain of magnetic observatories located along the west coast of Greenland. The investigation reveals that the equivalent ionospheric currents at the dayside polar cusp are located between two field‐aligned current sheets and the direction of the equivalent ionospheric current depends upon the directions of the two field‐aligned currents in a manner consistent with the assumption that the equivalent ionospheric current represents an ionospheric Hall current. Previous investigations have shown that the direction of the ionospheric cusp current is determined by the azimuthal component By of the interplanetary magnetic field (IMF). A similar relation therefore exists between the field‐aligned currents and By in the dayside cusp region. When By is positive, the ionospheric current (the DPY current) flows eastward, and the flow of the poleward field‐aligned current (the cusp region field‐aligned current) is away from the ionosphere in the northern hemisphere. When the polarity of By reverses, the flow directions of the ionospheric and field‐aligned currents reverse.